The present invention relates to a receiver mounted in a terminal used in a mobile communication system, and more particularly a receiver for receiving a spread spectrum signal and suitable for the terminal performing the intermittent receiving operation in the mobile communication system of the code division multiple access (CDMA) system.
In the terminal used in the mobile communication system and represented by a portable telephone, it is important to suppress the power consumption and extend the service time that the terminal can be operated. Accordingly, when the terminal is in the idle state, the terminal performs the intermittent receiving operation. In the intermittent receiving operation, the receiving and demodulation operation is made for the paging channel produced in the time division manner only during a time slot assigned thereto, and the receiving operation is suspended during sections of time slots not assigned thereto. During the suspension of the receiving operation, a power supply for unnecessary circuits is cut off and/or a processor is moved to a low-power consumption mode, so that the power consumption of the terminal is reduced.
In the portable telephone system of the CDMA system utilizing the spectrum spreading, the intermittent receiving operation named a slot mode is prescribed. In this case, the terminal is required to maintain synchronization of the PN (Pseud Noise) code during suspension of the receiving operation or attain high-speed re-synchronization upon resumption of the receiving operation. The requirement is to make the de-spreading using the replica PN code produced in the terminal upon demodulation of a received signal. This point is now described in detail.
FIG. 14 schematically illustrates a transmitter of a base station. This drawing is depicted to pay attention to the paging channel (PaCH) for the paging important to the intermittent receiving operation. A coded paging message is inputted as a data string to one input of an exclusive-OR gate (BS1). The other input of the gate is supplied with an output signal of a decimeter (BS2) which decimates an output signal of a PN-code-for-long-code generator (BS3) to the symbol rate of the data string. The code of the PN-code-for-long-code generator (BS3) has a period of 242−1 and a phase of code can be adjusted by a long-code mask bit. An output signal of the exclusive-OR gate (BS1) is changed to a scrambled paging message by the very long code series. In order to multiplex the scrambled symbol by a signal of another channel having a different function, the scrambled symbol is subjected to the spreading by orthogonal sequences in an exclusive-OR gate (BS4). The input symbol is spread to 64 chips by means of a series assigned to the paging channel of 64 kinds of orthogonal series named the Walsh function (BS5). The orthogonally spread output of the exclusive-OR gate (BS4) is multiplexed together with other pilot channel (PiCH), synchronization channel (SCH) and traffic channel (TCH) in a multiplexer circuit (BS6). Actually, binary codes are substituted with pulses having the bipolarity of 1 and −1 and are multiplied by a gain based on the power distribution of channels to be added. The multiplexed output signal of the multiplexer is separated into the I-signal and the Q-signal to be supplied to multipliers (BS7 and BS9), respectively. The multipliers (BS7 and BS9) are connected to different PN code generators including an I-channel pilot PN code generator (BS 8) and a Q-channel pilot PN code generator (BS10). Accordingly, I- and Q-signals of two-phase modulation signals are spread to four-phase modulation signals by two different series. The PN code used in the four-phase spreading is a series having a period of 215 named a short code. The four-phase spreading signals are supplied to baseband filters BS11 and BS12 in which the bandwidth thereof is retricted and are carrier-modulated by an quadrature modulator BS13 to be outputted. The foregoing is operation made by the base station. The long-code PN code generator (BS3) for scrambling and the I-channel and Q-channel pilot PN code generators (BS8, BS10) for 4-phase spreading are continuously operated irrespective of the structure of the slot for the paging channel time-divided by paging groups.
The terminal for receiving the signal obtains I- and Q-signals of the spread band by means of quadrature detection. Then, the signals are subjected to despreading using the short codes for the I- and Q-signals. Naturally, the short codes used in the terminal require the series synchronized with the base station. Since the short codes for spreading in the base station are produced continuously when the intermittent receiving operation is performed, it is necessary that the PN code generating means in the terminal is continuously operated apparently. In this case, the synchronization is maintained even during the suspension period of the intermittent receiving operation. Alternatively, even if the synchronization is not once maintained during the suspension period, there is no problem when the synchronization is made again before arrival of the assigned slot. However, the period of the short code is 215 and accordingly since new search for all of the phase space upon resumption of the receiving operation increases the receiving time rate of the intermittent receiving operation, it is not desirable. Hence, in order to enhance the effect of suppressing the power consumption in the intermittent receiving operation, the inventions described in JP-A-5-191375 and JP-A-8-321804 have been made.
In the former example, when the receiving operation is changed to the suspension state, a fixed frequency oscillation circuit is connected to a local spreading code generation circuit to drive the generation circuit by a free-running clock of the oscillation circuit, so that a phase difference is prevented from being increased. While a phase of the local spreading code delayed by a shift or deviation of a phase anticipated in the suspension time upon the resumption of the receiving operation is made progress gradually, the correlation peak is detected to make re-synchronization. In the invention, the local spreading code generation circuit is freely operated by itself during the suspension period and the power supply thereto cannot be cut off.
On the contrary, in the latter example, a state value of spreading code generation means at the time of next starting is set to register means to operate timer means. Upon re-starting due to time out of the timer means, the spreading code generation means is operated from the state value set in the register means. In this case, the power supply to the receiving circuit including the spreading code generation means can be cut off during the suspension period. However, in order to ensure that the expected state value is correct upon re-starting, the accuracy of timing by the timer means for controlling the suspension period is important. For example, when it is assumed that the suspension period is about several seconds and the chip rate of code is several mega chips per second, the timing accuracy of about one tens millionth is required in order not to produce any shift or deviation by one chip. As a reference oscillator used in the portable telephone, there is a voltage-controlled type temperature compensation crystal oscillator (hereinafter, abbreviated as VC-TCXO) and the VC-TCXO having the absolute accuracy of about 2.0 ppm is selected in view of the cost condition. In addition thereto, the frequency control referring to the received signal from the base station is performed, so that the timing accuracy required for the timer means can be obtained. That is, in the prior art of the latter example, it is required that the oscillator circuit of the high accuracy as the VC-TCXO is operated during the suspension period.
The current to be consumed in the VC-TCXO is, for example, about 1 μA and is larger than several μA of an IC for watch having an inferior accuracy thereto.